Conventionally, data has been stored in a storage (DAS: Direct Attached Storage) connected directly to a client computer. The data in the DAS is accessible only via the client computer connected directly thereto so that, if the client computer accesses the data in the DAS connected to another client computer, it is necessary for the client computer to access the data via the other client computer connected to the DAS.
As an amount of data to be stored in each of the client computers has increased exponentially due to the recent advancement of network technologies, the need to efficiently manage the data has also grown so that a storage system connected to a network, what is called a network storage, has been introduced as a replacement for the DAS. Since the network storage allows data to be shared by a plurality of client computers, an administrator can efficiently manage at data.
Examples of the network storage include a SAN (Storage Area Network) storage connected to a SAN to provide a block access and a NAS (Network Attached Storage) connected by an IP network, the Infiniband, or the like to provide a file access. Of the examples, the NAS has lower cost than the SAN storage and can easily be shared by a plurality of client computers by using a plurality of standard file access protocols such as NFS (Network File System) and CIFS (Common Internet File System) so that the market thereof has been expanding.
Since the network storage thus allows an administrator to simultaneously and effectively manage data shared by a plurality of client computers, it is used prevalently in the society. On the other hand, the shared data stored in the network storage is backed up by the administrator by using a tape drive or the like to be restored from the tape drive as required. However, since the tape drive is low in speed, the back-up process and restoration process using the tape drive require an enormous amount of time, which disadvantageously increases in direct proportion to an amount of the shared data. In particular, the capacity of the network storage has been expanded increasingly with the recent advancement of hard disk technology so that low-speed tape back-up is inefficient.
To solve the problem of the low-speed tape back-up, a method for holding a replica of shared data held in a network storage not in a tape drive but in another network storage connected to the network has been used recently. The method allows the creation of the replica of shared data and the restoration of the shared data at a speed higher than a method using a tape drive.
For example, a method for creating a copy of a file in real-time in a plurality of network storages is described in “VERITAS File Replicator 3.0.3, System Administrator's Guide, 2001 June”, published by VERITAS Software Corporation. In response to receipt of an updating file access request from a client, the VERITAS File Replicator simultaneously transfers the request to a first file system as a target of replication and to a plurality of file systems at replication destinations. Replication is performed with respect to an entire file system as a target.
On the other hand, U.S. Pat. Publication No. 2002/0120763 discloses a method in which a unification virtualizing switch having a special function for file replication termed a journaling file system transfers an updating file access request to a group of servers composing a primary virtualized-and-unified file system and to a secondary file server and thereby creates a copy of an entire file system. Upon receipt of a response from the group of servers composing the primary virtualized-and-unified file system, the journaling file switch returns the response to a client without waiting for the updating request to be processed at the secondary file server. This achieves a reduction in overhead for file copying.
In addition, a replication system for copying, in a distributed computer system, only the subtree of a computer system as a target of replication is disclosed in Japanese Patent Laid-Open No. H07(1995)-219830. In the replication system, a file access request from a client is transferred to a computer system having the file. If the file access request is an updating file access request, a computer system records the occurrence of a change in a log termed a replicated storage change record. The updating request in the replicated storage change record is copied at constant intervals or at prescheduled intervals in a computer system to a replication destination.
The foregoing technologies disclosed in “VERITAS File Replicator 3.0.3, System Administrator's Guide, 2001 June” and U.S. Pat. Publication No. 2002/0120763 can create a copy of a file in real-time between the plurality of network storages and between the plurality of virtualized-and-unified file systems. However, replication should be performed with respect to the entire file system.
Since the capacity of the network storage has been expanded increasingly as described above, if the creation of a file is limited to an entire file system, an increased back-up time presents a problem. In addition, the shared data include an essential and important file which should be backed up and a file which need not be backed up. However, since the creation of a copy is limited to an entire file system in each of the foregoing first and second well-known examples even when it is sufficient for the administrator to acquire a back-up of only the important file, not only the elongated back-up time but also the preparation of a network storage with a sufficient capacity at a copy destination presents problems.
Although the file replication method disclosed in Japanese Patent Laid-Open No. H07(1995)-219830 mentioned above has provided a method for creating a copy of the subtree of the file system, it is impossible to create the copy in real time in response to a file access request from a client. As a result, a back-up of the latest data is not held in such a case where trouble occurs, which presents a problem.
To solve the problems, it is necessary to provide a file replication method which can create a partial copy of a file system in real-time between a plurality of network storages.
It is also necessary to provide a file replication method which can create a partial copy of any of virtualized-and-unified file systems in real time between a plurality of virtualized-and-unified file systems.
In Japanese Patent Laid-Open No. H07(1995)-219830 mentioned above, it is also necessary for the administrator to manually maintain consistency when a trouble occurs in any of the plurality of virtualized-and-unified file systems while they are performing selective file replication or in the network to which the virtualized-and-unified file systems are connected and recovery from the trouble is performed. For manual maintenance of consistency, it is necessary for the administrator to examine whether or not each of the backed up files has already been copied.
To solve the problem, it is necessary to provide a method in which, when a trouble occurs in any of the plurality of virtualized-and-unified file systems while they are performing selective file replication or in the network and trouble recovery is performed, the consistency of a file and directory as objects to be copied is automatically maintained between the plurality of virtualized-and-unified file systems such that the replication is resumed.